U.S. patent application number 14/127547 was filed with the patent office on 2014-07-24 for method for hydraulically boosting an electric parking brake of a vehicle.
The applicant listed for this patent is Klaus Berger, Christian Kebbel, Peter Leska, Christop Maron, Klaus Dieter Pagel, Heinz-Anton Schneider, Andreas Strecker, Christian Strehle, Eduard Wiens, Jurgen Woywod. Invention is credited to Klaus Berger, Christian Kebbel, Peter Leska, Christop Maron, Klaus Dieter Pagel, Heinz-Anton Schneider, Andreas Strecker, Christian Strehle, Eduard Wiens, Jurgen Woywod.
Application Number | 20140202801 14/127547 |
Document ID | / |
Family ID | 46320981 |
Filed Date | 2014-07-24 |
United States Patent
Application |
20140202801 |
Kind Code |
A1 |
Berger; Klaus ; et
al. |
July 24, 2014 |
Method for Hydraulically Boosting an Electric Parking Brake of a
Vehicle
Abstract
A method for hydraulically boosting a vehicle electric parking
brake having a hydraulic service brake and an electric parking
brake. Application forces electically generated by the parking
brake function is superimposed on the boosting brake force
generated by a hydraulic boosting brake pressure provided by the
service brake to the brake actuator. When the parking brake is
actuated to generate a predetermined application force, the force
generated by the brake actuator is detected as the actual value, if
an actual value of the measured value is smaller than a first
target value a boosting brake pressure value is applied to the
brake actuator, and by means of the parking brake function an
adjustment function for the tension of the brake actuator to which
the hydraulic boosting brake pressure is applied is carried out to
achieve the predetermined application force.
Inventors: |
Berger; Klaus; (Frankfurt,
DE) ; Leska; Peter; (Dreieich, DE) ; Strehle;
Christian; (Stuttgart, DE) ; Strecker; Andreas;
(Frankfurt, DE) ; Wiens; Eduard; (Edelsberg,
DE) ; Schneider; Heinz-Anton; (Niedernhausen, DE)
; Maron; Christop; (Kelkheim, DE) ; Woywod;
Jurgen; (Morfelden, DE) ; Pagel; Klaus Dieter;
(Darmstadt, DE) ; Kebbel; Christian; (Rodgau,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Berger; Klaus
Leska; Peter
Strehle; Christian
Strecker; Andreas
Wiens; Eduard
Schneider; Heinz-Anton
Maron; Christop
Woywod; Jurgen
Pagel; Klaus Dieter
Kebbel; Christian |
Frankfurt
Dreieich
Stuttgart
Frankfurt
Edelsberg
Niedernhausen
Kelkheim
Morfelden
Darmstadt
Rodgau |
|
DE
DE
DE
DE
DE
DE
DE
DE
DE
DE |
|
|
Family ID: |
46320981 |
Appl. No.: |
14/127547 |
Filed: |
June 19, 2012 |
PCT Filed: |
June 19, 2012 |
PCT NO: |
PCT/EP2012/061639 |
371 Date: |
March 24, 2014 |
Current U.S.
Class: |
188/106P |
Current CPC
Class: |
F16D 65/14 20130101;
B60T 13/146 20130101; B60T 13/58 20130101; B60T 8/32 20130101; B60T
13/588 20130101; B60T 7/04 20130101; F16D 2121/02 20130101; B60T
13/741 20130101; F16D 2121/24 20130101; B60T 8/4872 20130101 |
Class at
Publication: |
188/106.P |
International
Class: |
B60T 13/58 20060101
B60T013/58; F16D 65/14 20060101 F16D065/14 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 20, 2011 |
DE |
10 2011 077 786.5 |
Jul 12, 2011 |
DE |
10 2011 079 040.3 |
Claims
1. A method for hydraulic boosting of an electric parking brake of
a vehicle comprising providing at least one brake actuator (2c,2d),
which for electric and hydraulic operation of the at least one
brake actuator (2c,2d) includes a hydraulic service brake function
and an electric parking brake function, wherein an application
force (F) electrically generated with the parking brake function is
superimposed with a boosting brake force generated by subjecting
the brake actuator (2c,2d) to a hydraulic boosting brake pressure
(P.sub.u) provided by the service brake function, operating of the
parking brake to generate a specified application force, detecting
the application force generated by the brake actuator (2c, 2d) as
the actual value (I.sub.actual) by means of a measurement variable
(I) representing the actual value, for a measurement of the actual
value (I.sub.actual) that is smaller than a first target value
(I.sub.1,target) representing a first target force value
(F.sub.1,target) of the application force, the brake actuator (2c,
2d) is subjected to a hydraulic boosting brake pressure (P.sub.u)
that corresponds to a specified target boosting brake pressure
value, (P.sub.u,target) and performing by means of the parking
brake function a further application function for further
application of the brake actuator (2c, 2d) that is subjected to the
hydraulic boosting brake pressure (P.sub.u) to achieve the
specified application force.
2. The method as claimed in claim 1, further comprising the steps
of to generate the hydraulic boosting brake pressure (P.sub.u) a
means of generating pressure (3), is controlled by means of the
service brake function.
3. The method as claimed in claim 2, further comprising in that the
target boosting brake pressure value (P.sub.u,target) is set to a
value at which the resulting boosting brake force corresponds to
the first target force value (F.sub.1,target), following the
generation of the hydraulic boosting brake pressure (P.sub.u)
corresponding to the target force value (F.sub.1,target) the means
of pressure generation (3) is switched off, and the boosting brake
pressure (P.sub.u) applied to the brake actuator (2c, 2d) is
hydraulically confined within the brake actuator (2c, 2d).
4. The method as claimed in claim 2, further comprising in that the
target boosting brake pressure value (P.sub.u,target) corresponds
to a specified inlet pressure target value (P.sub.u, inlet),
wherein the resulting boosting brake force is smaller than the
first target force value (F.sub.1,target), and following the
generation of the hydraulic boosting brake pressure (P.sub.u) the
means of generating pressure (3) is not switched off and the brake
actuator (2c, 2d) is further subjected to the boosting brake
pressure (P.sub.u).
5. The method as claimed in claim 3, further comprising in that for
further application of the parking brake the brake actuator (2c,
2d) is controlled by the boosting brake pressure (P.sub.u) by means
of the parking brake function until the actual value (I.sub.actual)
of the measurement variable (I) of the application force
corresponds to a target value (I.sub.2, target), which is smaller
than the first target value (I.sub.1,target) corresponding to the
first target force value (F.sub.1,target).
6. The method as claimed in claim 4, further comprising in that for
further application of the parking brake by means of the parking
brake function a displacement control is activated, wherein a brake
piston of the brake actuator (2c, 2d) that is subjected to the
boosting brake pressure (P.sub.u) is controlled to a target
position corresponding to the specified application force using a
stored displacement-force characteristic.
7. The method as claimed in claim 6, further comprising in that an
error message is generated if the brake piston target position is
not reached by a determined actual position.
8. The method as claimed in claim 4, further comprising in that the
further application function (2c, 2d) includes further application
of the brake actuator by means of the parking brake function and
thereby the actual value (I.sub.actual) of the measurement variable
(I) representing the application force generated by the brake
actuator (2c, 2d) is detected, and the further application is
terminated if the actual value (I.sub.actual) of the measurement
variable (I) reaches a second target value (I.sub.2,target)
corresponding to a second target force value (F.sub.2,target)
wherein the second target force value (F.sub.2, target) is reduced
relative to the first target force value (F.sub.1,target) by a
force value corresponding to the boosting brake pressure
(P.sub.u).
9. The method as claimed in claim 8, further comprising in that an
error message is generated if the actual value (I.sub.actual) of
the measurement variable (I) does not reach the second target value
(I.sub.2,target).
10. The method as claimed in claim 8, further comprising in that
prior to performing the further application, the brake actuator
(2c, 2d) that is subjected to the boosting brake pressure (P.sub.u)
is controlled by a small displacement amount in the direction of
releasing the parking brake by means of a displacement control.
11. The method as claimed in claim 1 further comprising in that the
hydraulic boosting brake pressure (P.sub.u) is reduced following
performance of the further application function.
12. The method as claimed in claim 1, further comprising in that
the brake actuator (2c, 2d) for performing the parking brake
function is driven by an electric motor (2d) and the motor current
of the electric motor (2d) is detected as the measurement variable
(I) representing the application force.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to German Patent
Application Nos. 10 2011 077 786.5, filed Jun. 20, 2011; 10 2011
079 040.3, filed Jul. 12, 2011; and PCT/EP2012/061639, filed Jun.
19, 2012.
FIELD OF THE INVENTION
[0002] The invention relates to a method for boosting an electric
parking brake of a motor vehicle.
BACKGROUND
[0003] A combined vehicle brake is known from DE 102008051350 A1
that describes a hydraulically operable service brake and an
electromechanically operable parking brake, also known as a fixing
brake, wherein a hydraulic working pressure chamber within a brake
housing is bounded by a brake piston, which can be subjected to
hydraulic pressure medium on the one hand to perform service
braking and on the other hand can be operated along a longitudinal
axis of a piston by means of an irreversible transmission to
achieve parking braking, and wherein the irreversible transmission
converts the rotational motion of an electric motor into a
translational motion of the brake piston to perform the parking
brake process and holds the brake piston in the activated position
by the self-locking of the transmission.
[0004] Such a parking brake system also includes an electronic
controller, which generally corresponds to the driving dynamics
controller (DDC controller) of the motor vehicle and contains the
known hydraulic and electric components as well as additionally two
full bridges for independent control of two direct current motors
of the two parking brakes on the rear wheels of the motor
vehicle.
[0005] With such a parking brake system, customer and safety
requirements regarding price, performance, noise, safety,
availability and comfort are to be achieved, which are often in
conflict with each other.
[0006] The design of such systems must therefore be selected such
that under normal ambient conditions, e.g. regarding the voltage of
the on-board electrical system and the temperature of the electric
motors of the parking brake, the necessary application force, which
must reach a minimum value, can be reliably applied. For cost
reasons, the application force is not directly measured for this
purpose, but is determined indirectly from other parameters
representing the application force. Thus e.g. for applying the
electric parking brake described above a predetermined maximum or
switch-off current of the electric motor is used as a switch-off
criterion, whereby it is assumed that the target application force
of the limit value is achieved as a target force value.
[0007] However, this method cannot guarantee safe stopping of the
vehicle in all situations, because e.g. at increased temperature
and/or low operating voltage the desired switch-off current is no
longer achieved.
[0008] To solve the problem it is known e.g. from DE 10 2010 029
391 A1 to use the vehicle hydraulics in support of achieving the
necessary application force. For this purpose, prior to the
application process on the parking brake, it will be decided
whether a hydraulic boost is necessary using existing temperature
values on the brake actuators and using acceleration values of the
wheels. According to the known method according to DE 10 2010 029
391 A1, the hydraulic boosting brake pressure is generated such
that in the event of operating the parking brake pressure medium is
initially taken from the front wheel brakes and transferred to
build up a boosting brake pressure in the parking brakes on the
rear wheels, and only then is application of the parking brake
performed.
[0009] With the known method of hydraulically boosting the parking
brake function, prior to applying the parking brake it must be
decided whether a hydraulic boost is necessary. This leads to the
disadvantage that too often a hydraulic boost is performed although
it would not have been necessary. Such behavior represents a
significant loss of comfort for the driver.
[0010] The object of the invention is to specify a method for the
hydraulic boosting of an electric parking brake of a vehicle of the
above-mentioned type, with which the disadvantages can be
avoided.
[0011] This object is achieved by a method with the features
described herein.
SUMMARY AND INTRODUCTORY DESCRIPTION OF THE INVENTION
[0012] The method according to the invention for hydraulic boosting
of an electric parking brake of a vehicle comprising at least one
brake actuator, which includes a hydraulic service brake function
and an electric parking brake function for electric and hydraulic
operation of the at least one brake actuator, wherein an
application force electrically generated with the parking brake
function is overlaid with a boosting brake force generated by
subjecting the brake actuator to a hydraulic boosting brake
pressure provided by the service brake function, is characterized
according to the invention such that when the parking brake is
operated to generate a specified application force, the application
force generated by the brake actuator is detected as an actual
value by means of a measurement variable representing its value, in
the event of an actual value of the measurement variable that is
smaller than a first target value representing a first target force
value of the application force, the brake actuator is subjected to
a hydraulic boosting brake pressure corresponding to a specified
target boosting brake pressure value, and a further application
function is performed by means of the parking brake function to
further apply the brake actuator that is subjected to the hydraulic
boosting brake pressure to achieve the specified application
force.
[0013] With the method according to the invention, a hydraulic
boost will only be performed or requested if it is actually
necessary, i.e. the actual value of the measurement variable
representing the application force, which preferably represents the
motor current of the electric motor of the parking brake, does not
reach a switch-off current value or a current limit value as the
target value corresponding to a first target force value. Moreover,
the energy expended to generate the hydraulic boosting brake
pressure is low because the brake piston has already been moved
close to the desired end position by the parking brake
function.
[0014] A hydraulic pump is used to generate the hydraulic boosting
brake pressure in an obvious manner as a means of generating
pressure and is controlled by means of the service brake function.
Because the brake piston of the brake actuator has already been
moved close to its end position by means of the parking brake
function, the volume to be supplied by the hydraulic pump remains
small, i.e. the energy consumption of the hydraulic pump is
low.
[0015] In one embodiment of the invention the target boosting brake
pressure value is set to a value at which the resulting boosting
brake force corresponds to the first target force value, i.e. the
necessary application force, wherein following the generation of
the hydraulic boosting brake pressure corresponding to the target
force value the means of pressure generation is switched off and
the boosting brake pressure applied to the brake actuator is
hydraulically confined in the brake actuator. In this way the brake
piston of the brake actuator is subjected to the boosting brake
pressure corresponding to the desired final value of the
application force in order to be able to perform the further
application.
[0016] The subsequent further application of the parking brake
takes place according to another embodiment of the invention by
controlling the brake actuator that is subjected to the confined
boosting brake pressure by means of the parking brake function,
until the actual value of the measurement variable of the
application force corresponds to a target value that is smaller
than the first target value corresponding to the first target force
value. The target value can thereby be selected such that a slight
movement of the brake piston of the brake actuator is initiated,
i.e. the electric motor provided to drive the brake actuator
performs some revolutions. Rapid and smooth-running driving of the
brake actuator to the brake disks is achieved by this slight
control of the brake actuator.
[0017] According to another embodiment of the invention the target
boosting brake pressure value corresponds to a target inlet
pressure value at which the resulting boosting brake force is
smaller than the first target force value, wherein following the
generation of the hydraulic boosting brake pressure the means of
generating pressure is not switched off, so that the brake actuator
continues to be subjected to the generated boosting brake pressure.
According to the development, displacement control is subsequently
activated for further application of the parking brake by adjusting
the brake piston of the brake actuator, which is subjected to the
boosting brake pressure of the parking brake function, to a
position corresponding to the specified application force target
using a stored displacement-force characteristic. If the target
position is not achieved with the brake piston with the further
application of the parking brake by determining its actual
position, an error message is generated for the vehicle driver.
[0018] Instead of the displacement control, according to one
embodiment of the invention the further application of the parking
brake can also be performed by detecting the actual value of the
measurement variable, which represents the application force
generated by the brake actuator, during the further application of
the brake actuator by means of the parking brake function and
ending the further application if the actual value of the
measurement variable reaches a second target value corresponding to
a second target force value, at which the second target force value
is reduced relative to the first target force value by a force
value corresponding to the boosting brake pressure. Preferably,
with the embodiment an error message is also generated if the
actual value of the measurement variable does not reach the second
target value.
[0019] In an advantageous manner, prior to performing the further
application using the second target value of the measurement
variable the brake actuator that is subjected to the boosting brake
pressure can be controlled by means of a displacement control by a
small displacement in the releasing direction of the parking brake
in order to achieve thereby a defined position for the subsequent
further application.
[0020] According to one advantageous development of the invention,
the hydraulic boosting brake pressure is reduced after performing
the further application function.
[0021] In order to perform the parking brake function the brake
actuator is driven by an electric motor, so that it is convenient
to detect the motor current of the electric motor as the
measurement variable representing the application force.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The method according to the invention is explained in detail
and described below with reference to the accompanying figures. In
the figures:
[0023] FIG. 1 shows a schematic block circuit diagram of a brake
system for performing the method according to the invention,
[0024] FIG. 2 shows a schematic process diagram for explaining a
first exemplary embodiment of the invention,
[0025] FIG. 3 shows a schematic process diagram for explaining a
second exemplary embodiment of the invention, and
[0026] FIG. 4 show a schematic process diagram for explaining a
third exemplary embodiment of the invention.
DETAILED DESCRIPTION
[0027] FIG. 1 shows a brake system 1 of a motor vehicle with a
controller 20 of a driving dynamics control system (DDC System), to
which the sensor data of a DDC sensor 21 and state signals of a
parking brake switch 10 are supplied, wherein the application or
release of a parking brake by the driver of the motor vehicle can
be initiated with the parking brake switch 10. For this purpose the
controller 20, besides performing a service brake function by means
of the brake system, additionally performs a parking brake function
by controlling components of a parking brake that are explained
below.
[0028] The controller 20 controls a brake system 1, wherein FIG. 1
only shows a brake circuit I for a front left wheel 11 and a rear
right wheel 12 connected to a brake cylinder (tandem master
cylinder) 9; a second brake circuit II is correspondingly designed
and is likewise connected to the brake cylinder 9 and is only
illustrated in outline. Wheel brakes of the wheels 11 and 12 with a
brake caliper 2a or 2b are each connected via a hydraulic line to
the brake circuit I, wherein the rear wheels 12 are each designed
as a parking brake with a brake actuator 2c, which are operated by
direct current motors 2d independently of a hydraulic pressure
application.
[0029] To perform the parking brake function, the controller 20 is
connected to the brake actuators 2c in order to control direct
current motors 2d. The rotary motion of the direct current motors
2d is converted into a linear movement of a brake piston of the
brake actuator 2c via a transmission, so that the torque of the
direct current motor 2d acts on the brake caliper 2b via the brake
piston as an application force F. Following switching off of the
motor current I the generated application force F is maintained
because of the self-locking of the transmission.
[0030] The application force F itself is not measured. When
applying the parking brake the direct current motor 2d is driven in
the application direction with a positive applied voltage via an H
bridge, i.e. for example by the controller 20. After the motor has
run up, the current rapidly reduces to a minimal value, the
so-called no-load current, while the maximum revolution rate is
set. After the linings have come into contact with the brake disks,
i.e. after overcoming the air gap, a force builds up and the
current increases. The brake piston of the brake actuator is now
driven by the direct current motor 2d until a specified, defined
target value I.sub.target of the motor current I is reached as the
actual value I.sub.actual or is exceeded. The application force
F.sub.target associated with the corresponding current value
I.sub.Target is then reached and the direct current motor 2d is
switched off.
[0031] For setting the necessary application force F.sub.target the
motor current I of the direct current motor 2d is measured and the
voltage applied to the direct current motor is specified by the
controller 20, which is connected to a not illustrated H bridge,
according to the necessary direction, depending on whether
application or release is to be carried out. Once a defined target
value I.sub.target of the motor current I is reached as the actual
value I.sub.actual or is exceeded, the direct current motor 2d is
switched off. Said target value corresponds to the necessary
application force F.sub.target here.
[0032] The target values I.sub.target of the motor current I are
determined by measuring the properties of the brake actuators 2c by
experiment and storing the corresponding current values used as
target values I.sub.target in a characteristic of the controller
20.
[0033] The position of the brake piston of the brake actuator 2c or
the revolution rate of the direct current motor 2d is likewise not
detected directly but is estimated by means of a model.
[0034] The brake system 1 includes a brake booster 13 connected to
the brake master cylinder 9 and a reservoir 5 for the brake fluid
or the hydraulic fluid. The brake cylinder 9 generates at the
outlet a brake pressure corresponding to a brake pedal 16 that is
connected to the brake booster 13 and that is operated by a driver.
The brake pressure is supplied via an opened isolating valve 15 to
an opened inlet valve 7a or 7b respectively on the inlet side so
that a corresponding hydraulic brake pressure can build up at the
wheels 11 and 12 by means of the brake calipers 2a and 2b. The two
inlet valves 7a and 7b are normally open. The brake booster 13 with
the brake master cylinder 9 is connected to the controller 20.
[0035] A normally closed outlet valve 8a or 8b connects the brake
calipers 2a or 2b to a low pressure reservoir 14, which for its
part is connected to a hydraulic pump 3 on the suction side and can
be connected via a change-over valve 6 to the brake master cylinder
9.
[0036] The hydraulic pump 3 is provided for the brake calipers 2a
and 2b in order, e.g. in the case of an ABS or DDC application, to
force back out the brake medium that is displaced in the low
pressure reservoir 14 during a pressure reduction or in order to
generate a hydraulic boosting brake pressure P.sub.u at the rear
wheels 12 when triggering the parking brake, if the necessary
application force F.sub.e,target cannot be generated. The brake
piston of the brake actuator 2c is subjected to the hydraulic
boosting brake pressure P.sub.u and is superimposed with the
application force F.sub.e electrically generated by means of the
direct current motor 2d.
[0037] The hydraulic pump 3 is driven by an electric motor 4, which
is itself controlled by the controller 20 using pulse width
modulation (PWM). The electric motor 4 is controlled here such that
the hydraulic pump 3 can build up a brake pressure on the high
pressure side by drawing in brake fluid on the suction side.
[0038] Thus during control of the hydraulic pump 3 in stepper motor
mode, brake fluid can be sucked out of the pressure chamber of the
brake caliper 2a or 2b, the change-over valve 6 and the inlet valve
7a or 7b are closed, while the outlet valve 8a or 8b is opened to
make the connection to the pressure chamber.
[0039] A first exemplary embodiment of the invention is described
below in combination with the process diagram according to FIG. 2
and the brake system according to FIG. 1. Wherein the description
related to the brake circuit I also relates to the brake circuit
II.
[0040] Following the start (step SI: "Start") of the process and
the detection of a shut parking brake switch 10 by the controller
20 according to step S2 ("parking brake closed"), application of
the brake caliper 2b to the rear wheel 12 takes place with step S3
by the controller 20 controlling the direct current motor 2d
accordingly by means of the parking brake function. The necessary
application force F.sub.1,target corresponds to a specified target
value I.sub.1,target of the motor current of the direct current
motor 2d (I.sub.1,target=F.sub.1,target). Therefore in step S4 the
actual value I.sub.actual of the motor current I is detected and
compared with the target value I.sub.1,target. If the target value
I.sub.1,target is reached or exceeded by the actual value
I.sub.actual the process is terminated with step S8 ("End").
[0041] In another case the controller 20 requests a hydraulic
boost, i.e. a hydraulic boosting pressure P.sub.u, to which the
brake piston of the brake actuator 2c is subjected, is generated by
the hydraulic pump 3 according to step S5 ("Generate P.sub.u") by
means of the service brake function. The target value
P.sub.u,target of the boosting pressure P.sub.u to be generated
corresponds here to the necessary application force F.sub.1,target.
The boosting brake pressure P.sub.u corresponding to the target
value P.sub.u,target is only built up at the parking brakes of the
rear wheels 12. For this purpose the inlet valve 7b is opened and
the outlet valve 7a of the brake caliper 2a on the front wheel 11
is closed and the outlet valves 8b of the brake caliper 2b of the
rear wheel 12 also remains closed. Following the buildup of
pressure at the parking brakes, the hydraulic boosting brake
pressure P.sub.u is confined within the brake actuator 2c by the
inlet valve 7b being closed. The hydraulic pump 3 is then switched
off.
[0042] Subsequently with step S6 a further application of the brake
caliper 2b of the parking brake is performed by controlling the
direct current motor 2d until its motor current I reaches a target
value I.sub.2,target that is lower than the first target value
I.sub.1,target The second target value I.sub.2,target is selected
such that a small rotary motion of the direct current motor 2d is
achieved, so as to cause thereby a slight associated driven
movement of the brake piston of the brake actuator 2c on the brake
caliper 2b under the effect of the confined boosting brake pressure
P.sub.u. The built up application force F is maintained because of
the irreversible transmission even following the opening of the
valves 7b and 8b to reduce the pressure according to step S7
("Reduce P.sub.u).
[0043] The second method for hydraulic boosting of the electric
parking brake of the rear wheels 12 starts according to FIG. 3 with
steps SI to S4, which correspond to those according to FIG. 2. If
according to step S4 the actual value I.sub.actual of the motor
current does not reach the target value I.sub.1,target
corresponding to the target force value F.sub.1,target, i.e. the
necessary application force F.sub.1,target is not set, likewise a
hydraulic boost is requested by the controller 20. For this
purpose, following step S5 (generation of P.sub.u) a hydraulic
boosting brake pressure P.sub.u is generated by means of the
hydraulic pump 3 with a target pressure P.sub.u,target that
corresponds to a target inlet pressure value P.sub.inlet, whose
value is smaller than the target force value F.sub.1,target. For a
target force value F.sub.1,target of e.g. 140 bar, e.g. 80 bar can
be selected as the target inlet pressure value P.sub.inlet.
[0044] The boosting brake pressure P.sub.u corresponding to the
target inlet pressure value P.sub.inlet is only built up at the
brake piston of the brake actuators 2c of the rear wheels 12. The
hydraulic pump 3 continues to run.
[0045] The further application is performed with the following step
S6 ("Further application with displacement control to target
position of the brake piston") by driving the brake pistons of the
brake actuators 2c at the rear wheels 12, which are subjected to
the hydraulic brake pressure P.sub.u, to a position corresponding
to the necessary application force, i.e. the target force value
F.sub.1,target, by means of displacement control using a
displacement-force characteristic stored in the controller 20.
[0046] Here we are starting from the position at the end of the
application process according to step S3. Starting from here the
displacement control uses a target position value having a slightly
larger value than the position at the end of the application
process. For the target position it is assumed that the brake
caliper 2b is applied with a correspondingly greater force. The
displacement controller seeks during further application to adjust
the brake piston of the brake actuator 2c in the clamping direction
to the target position. On achieving the target position, following
step S7 ("Target position reached") the hydraulic boosting brake
pressure P.sub.u is reduced in step S9 ("Reduce Pu), i.e. the
hydraulic pump 3 is shut off. In a different case, i.e. on not
achieving the target position, in step S8 ("Display error message")
an error message for display to the driver is generated and the
boosting brake pressure P.sub.u is reduced with step S9.
[0047] The two methods for hydraulic boosting of the electric
parking brake explained in combination with FIGS. 2 and 3 have the
advantage that the hydraulic boost is only requested or performed
if it is also actually necessary. Furthermore, the volume of
pressure medium to be generated by the hydraulic pump 3 is very
small because, as a result of the application process performed by
the parking brake function according to step S3, the brake piston
of the brake actuators 2c has already been moved close to the
necessary end position before the hydraulic boosting brake pressure
acts.
[0048] Another advantage is that the described method can also be
used for further application of an overheated parking brake during
the cooling phase.
[0049] The last exemplary embodiment of a method for hydraulic
boosting of the electric parking brake according to FIG. 4 differs
from that according to FIG. 3 by the further application function
used.
[0050] For the further application function, in step S6 a target
value I.sub.2,target for the motor current I of the direct current
motor 2d is used that corresponds to a second target force value
F.sub.2,target, which is reduced relative to the first target force
value F.sub.1,target used in steps S3 and S4 by a force value
corresponding to the boosting brake pressure P.sub.inlet. The brake
piston of the brake actuator 2c subjected to the boosting brake
pressure P.sub.pre is now applied by the controller 20 using the
direct current motor 2d until the actual value I.sub.actual of the
motor current I reaches or exceeds the target value I.sub.2,target;
the direct current motor 2d is then switched off.
[0051] The reduced target force value I.sub.2,target is also taken
from a characteristic stored in the controller 20. Such
characteristic fields are determined for defined hydraulic inlet
pressures that are applied to the brake piston of the brake
actuator 2c.
[0052] For hydraulic inlet pressures between the values for which
characteristic fields exist, the corresponding target value
I.sub.2,target for the motor current I can be interpolated.
[0053] If the target value I.sub.2,target is not reached by the
motor current I, according to step S7 ("Display error message") an
error message is generated for the driver and the process is
terminated (step S9: "End") following reduction of the hydraulic
boosting brake pressure P.sub.u according to step S8 (Reduce
P.sub.u).
[0054] Prior to the further application of the parking brake
according to step S6, initially the brake actuator 2c that is
subjected to the boosting brake pressure P.sub.u can be controlled
by a small displacement amount in the direction of releasing the
parking brake by means of a displacement control in order to
achieve a defined position for the subsequent further
application.
[0055] The last described method according to FIG. 4 also has the
advantage that a hydraulic boost is only performed if this is
actually necessary. Likewise the volume to be supplied by the
hydraulic pump 3 for the hydraulic boosting pressure is small
because the brake piston of the brake actuator 2d has already been
moved close to the necessary end position according to steps S3 and
S4.
[0056] While the above description constitutes the preferred
embodiment of the present invention, it will be appreciated that
the invention is susceptible to modification, variation, and change
without departing from the proper scope and fair meaning of the
accompanying claims.
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